Currently, you can upload up to 30 pages of a document to the whiteboard.

How do I share files I've uploaded as message attachments?

All file types can be shared using the chat panel on the right side of the screen. (If you don’t see the chat panel, make sure you’ve switched out of the full video or full whiteboard mode.) Simply click the paper clip icon at the bottom right corner of the panel and select the file you’d like to share from your computer.